1. Field of the Invention
The present invention generally relates to a material having a thin film of titanium dioxide deposited on a substrate and a method of producing the same.
2. Description of Related Art
Semiconductor substrates commonly employ metal oxide films. In recent years, titanium dioxide (TiO2) has become desirable because it is believed to have desirable characteristics. More specifically, TiO2 has been believed to be desirable for a variety of reasons, including that it is a stable material, insoluble in water, attacked only by acids and alkalis, and is non-toxic.
Four natural polymorphs for TiO2 are known: 1) anatase, 2) rutile, 3) brookite, and 4) TiO2 (B). Anatase TiO2 is has a single-phase, crystalline, tetragonal structure. Rutile also has a tetragonal structure, but comprises both an amorphous and a crystalline structure. Brookite has an orthorhombic structure, and TiO2 (B) has a monoclinic structure.
A solar cell is a semiconductor that converts light photons into electricity. Solar cells are made by joining p-type and n-type semiconducting material. The positive and negative ions within the semiconductor provide the environment necessary for an electrical current to move through a solar cell. A solar cell photogenerates charge carriers (electrons and holes) in a light-absorbing material and separates the charge carriers. Preferably, a solar cell will separate the charge carriers to a conductive contact that will transmit the electricity.
Anatase TiO2 is used as a semiconductor film in dye-sensitized solar cells. Typically, the anatase TiO2 film is prepared by deposition from a colloid produced by a sol-gel method, resulting in a relatively thick film having a structure that is at least partially amorphous. In order to achieve crystallization, the film is subject to a hydrothermal treatment at 230° Celsius.
Semiconductor substrates employing metal oxide films have also been used in gas sensors. Gas sensors are typically fabricated as sintered porous pellets, or thick films, in which the resistance of the material depends largely on gas adsorption. Both n-type and p-type semiconductors may be used in gas sensors, but n-type are generally preferred because a resistance decrease with concentration is preferable over a resistance increase. In stoichiometric n-type oxides, carbon monoxide (CO) can inject electrons into the conduction band, which results in increasing the conductivity of the material; the CO interacts directly with the oxide rather than adsorbed oxygen.
Although titanium dioxide was thought to be desirable for the above applications, it is typically thicker than desired and methods of producing it have resulted in inconsistent densities. For example, the sol-gel method of deposition is known to create films having thicknesses in the range of 2 to 4 micrometers.
Additionally, titanium oxide films have typically contained contaminants, such as carbon. Furthermore, the films have typically been produced having a rutile phase, with an anatase phase being achieved only upon high temperature annealing, which may cause durability to be weakened. For example, use of chemical vapor deposition (CVD) results in the deposition of a rutile titanium dioxide film, which can be converted to anatase film by only by high temperature annealing. CVD also may result in poor adhesion of the film to the substrate.
In view of the above, there exists a need for a thin film titanium dioxide that is preferably in an anatase phase upon deposition, which is relatively thin and has good adhesion and durability properties.